1. Field of the Invention
This invention relates generally to high power semiconductor devices and more particularly to high power heterojunction bipolar transistors utilized for microwave applications.
2. Description of Related Art
Heterojunction bipolar transistors (HBTs) are generally known and comprise one or more HBT cells typically fabricated in gallium arsenide (GaAs). Microwave circuits including HBTs can be used for power, low noise and linear amplification as well as power generation in oscillators.
In many respects the GaAs HBT resembles silicon bipolar junction transistors inasmuch as both devices are minority carrier, vertically oriented structures in that vertical current flow from the emitter to collector is modulated by a thin base layer where the injected carriers become minority charges. The emitter of an HBT is fabricated with wider band gap material than that of the base. Although the emitter-base junction is a forward biased p-n junction very similar to a silicon bipolar junction transistor, the presence of a band gap discontinuity at the heterojunction acts to restrict the flow of charges from the base into the emitter while allowing charge flow in the other direction. This imbalance results in current gain and is almost independent of the emitter and base doping levels. The microwave power HBT makes use of this feature. Moreover, the use of a heavily doped base layer is a key to obtaining high power density operation. Device designs can be optimized for high current and high voltage operation while maintaining sufficient microwave power gain for efficient operation. A low base sheet resistance makes it easier to modulate charges over a larger area under the emitter contact. The emitter utilization factor is high enough to use emitter strip widths of several micrometers for microwave frequency operation. As a result, the area under the emitter contact essentially becomes a uniform, large current channel. This is in contrast to the silicon bipolar junction transistor wherein the higher base sheet resistance restricts the emitter current flow to areas around the emitter periphery. For more detailed treatment of the heterojunction bipolar transistor the reader is referred to "GaAs HBTs for Microwave Integrated Circuits," B. Bayraktaroglu, Proc. IEEE, Vol. 81, No. 12, December 1993, pp. 1762-1784.